Jimmy Snethen, age 11, of Newport News, Virginia, for his question:
What exactly is a fog?
Believe it or not, a fog is merely a grounded cloud. Its kinfolk float aloft above the earth, blown by the breezes, changing shape as they sail from here to there. A fog tends to squat where it is because as a rule there is no wind to waft it away. Both fogs and clouds are made from the same misty moisture suspended in the filmy air.
The basic ingredients needed to form a fog are air and water vapor. The energy used to create it is provided by changing temperatures. In most cases, the weathery event occurs when a mass of moist air becomes cooler. This can happen almost anywhere on chilly mornings. It happens most often along coasts where mild moist sea breezes clash with masses of cooler air above the land.
Water vapor is an invisible gas made of separate water molecules. There is always a certain amount of vapor in the air, though the percentage is lower over arid regions and higher above tropical rain forests. Its separate molecules mix and mingle with the gaseous molecules of the air, occupying the same space.
However, there is a limit as to how much vapor the atmosphere may contain. And this limit is governed by strict rules of temperature. Heat, as we know, is a form of energy. In warm air, the gaseous molecules use this energy to spread farther apart. Hence, warm air tends to be thinner and lighter than cool air. And warm air can contain a larger percentage of water vapor than cool air.
This basic rule is very precise. Meteorologists measure the vapor content in grams per cubic meter. For example, if the temperature is 86 degrees Fahrenheit, a cubic meter of air can hold up to 30.4 grams of moisture and no more. Many a summer day reaches this temperature when the maximum moisture creates a muggy condition called high humidity. As a rule, things cool off during the night. But as the temperature drops, so does the allowable quota of vapor in the air.
And what happens then is likely to create an early morning fog. Suppose the pre dawn temperature drops to 68 degrees. A cubic meter of air at this temperature is saturated when it contains 17.31 grams of vapor. This means that every cubic meter of early morning air now contains a surplus of 13.09 grams of moisture. And something must be done to get rid of it.
The surplus moisture in the cooler air may form an early morning shower of raindrops. On the other hand, the extra water molecules may form tiny droplets of misty moisture, small and light enough to hang suspended in the air. The moisture is now in visible liquid form and what we see is a misty fog.
As the sun climbs higher, the air becomes warmer again. As its temperature rises, the air can begin to re absorb more vapor. The morning fog evaporates, or burns off, as its liquid droplets are changed back into gaseous vapor molecules. Other types of fog form on mountain slopes and along the shores when masses of cool dry air chill masses of mild moist air. In each case, the fog is a misty cloud of water droplets.